1. Field of the Invention
The present invention relates to a gas turbine which is preferably used for a power plant or the like, and in particular, a turbine blade equipped with a cooling structure.
2. Description of Related Art
To improve heat efficiency of an industrial gas turbine used for a power plant or the like, it is effective that the temperature of a combustion gas (fluid) for operation at an inlet of the turbine is increased. However, since the heat resistance performance of each of the members which are exposed to the combustion gas, such as moving blades, stationary blades, and turbine blades, is limited by the physical characteristics of the materials used in the members, the temperature of the inlet of the turbine cannot be simply increased.
To solve the above problem, since the turbine blades are cooled by a cooling medium such as cooling air or the like, and simultaneously, the temperature of the inlet of the turbine is increased, the heat resistance performance of the turbine blades is maintained to improve the heat efficiency.
Examples of cooling methods for the turbine blade include a convection cooling method and an impingement cooling method in which the cooling medium passes through the inside of the turbine blade, and a film cooling method in which the cooling medium is injected to the outside surface of the turbine blade to form a film of the cooling medium.
Furthermore, a structure of a conventional moving blade (turbine blade) is explained below with reference to FIGS. 4A and 4B.
FIG. 4A is a perspective view explaining an example of a structure of moving blade member 50 and FIG. 4B is a cross-sectional view along the line C—C in FIG. 4A of a top portion TP which is a tip portion of moving blade 51. Moving blade 51, and tip squealers 54a and 54b (protrusion parts) which are provided on the top portion TP are shown in FIGS. 4A and 4B.
As shown in FIG. 4A, moving blade 51 is disposed upright on platform 55 which is provided on engaging part 56 fixed to a turbine rotor (not shown). At both side surfaces, high pressure side blade surface 53 (outside surface) and low pressure side blade surface 52 (outside surface) are provided. At high pressure side blade surface 53, a high pressure combustion gas flows due to the rotation of moving blade 51, and at low pressure side blade surface 52, a low pressure combustion gas at a pressure lower than the combustion gas flowing at high pressure side blade surface 53 flows.
As shown in FIG. 4B, at the top portion TP which is the tip portion of moving blade 51, the protrusion parts, called tip squealers 54a and 54b, having a height h2 are provided along both blade surfaces 52 and 53 of moving blade 51. These tip squealers 54a and 54b are used as portions to be abraded when the top portion TP makes contact with a wall surface at the opposite side when the turbine is started.
Moving blades 51 are arranged in a path of the combustion gas which blows out from a combustor (not shown). The path is composed of a wall surface of platform 55 and an inner wall surface (not shown) of a casing which forms the exterior of the turbine. The casing is a separating ring.
When the gas turbine is started, a high temperature gas collides against moving blade 51, resulting in the heat expansion of moving blade 51. The stationary blades of course also undergo heat expansion. However, since the casing does not make direct contact with high temperature gas, the casing undergoes heat expansion more slowly than these moving and stationary blades. Therefore, the casing cannot undergo heat expansion in response to the heat expansion of each blade. In this condition, since moving blades 51 and the like are rotated together with a rotation axis in the casing, the top portion TP of moving blade 51 may be abraded by making contact with the inner wall surface of the casing. This phenomenon is called “tip rubbing” and occurs because the top portion TP of moving blade 51 and the inner wall surface of the casing are closely formed so as to prevent pressure leakage from a space between the top portion TP and the inner wall surface.
Since tip squealers 54a and 54b, which are provided as the portions to be abraded or for holding pressure have a sufficient height h2, if tip rubbing is generated, the height h2 sufficiently corresponds to the portion to be abraded.
However, if such a relatively large concaving formed by tip squealers 54a and 54b is provided at the top portion TP of moving blade 51 which has a high temperature, disadvantages are generated in many respects. For example, since the top portion TP is separated from the surface to be cooled, it is difficult to cool the top portion TP. Therefore, the durability of the top portion TP with respect to the operating the turbine may be decreased by burnout of the top portion TP and the further generation of cracking.
To solve the above problems, the top portion TP of moving blade 51 has a structure as shown in FIGS. 5A and 5B. FIGS. 5A and 5B are cross-sectional views showing the top portion TP of moving blade 51. FIG. 5A shows a condition before the generation of tip rubbing and FIG. 5B shows a condition after the generation of tip rubbing.
FIG. 5A shows tip squealer 54 (protrusion part) which is formed along high pressure side blade surface 53, and plural holes 56 and 57 which are provided on the top portion end surface. Holes 56 and 57 are formed in two directions, respectively. One hole is formed so as to penetrate tip squealer 54 containing a step portion having a height of h3 which is lower than the height of tip squealer 54a shown in FIG. 4B. The other hole is formed at the end surface of top portion TP in which a portion corresponding to tip squealer 54b is removed.
Each of holes 56 and 57 communicates with cavity R in moving blade 51, and cooling medium inflowing into moving blade 51 is taken up from upstream opening portions 56b and 57b of holes 56 and 57 and is blown out from downstream opening portions 56a and 57a. As a result, the cooling medium blown out from the opening portions cools the top portion TP, blade surfaces 52 and 53, and the inner wall surfaces, which face the blade surfaces, of the casing.
Upstream opening portions 56b and 57b and downstream opening portions 56a and 57a are holes having the same cross-sectional area about 1 mm in diameter, and are generally formed by electric discharge machining, laser beam machining, or the like.
According to the above constitution, since the cooling medium blown out from holes 56 and 57 is used to cool the top portion TP and the like, the thermal stress of tip squealer 54 is relaxed and is prevented from burning out and cracking. Furthermore, since the height of tip squealer 54 is lower than the height of tip squealer 54b shown in FIG. 4B and a tip squealer is provided at only one side of moving blade tip 51, a thermal stress concentration is avoided to a large extent and burnout and cracking are prevented.
However, in moving blade 51 as a conventional turbine blade, when tip rubbing is generated on the top portion TP, the periphery of each of holes 56 and 57 of the cooling medium is abraded, resulting in a problem wherein these holes 56 and 57 are covered. This is because the member of the top portion TP is deformed by abrasion and burrs, for example, remain at the periphery of each of holes 56 and 57 in the deformed member.
The condition after the generation of tip rubbing is explained below with reference to FIG. 5B. When the top portion TP of moving blade 51 makes contact with the inner wall surfaces of the casing due to heat expansion, the top portion TP is gradually abraded removing the portion having a height α. Simultaneously, holes 56 and 57 formed at the end surface of the top portion TP are abraded forming downstream opening portions 56a′ and 57a′ whose ends are shifted downward. At the same time, the periphery of each of downstream opening portions 56a′ and 57a′ is abraded generating burrs. The cross-sectional area of each of downstream opening portions 56a′ and 57a′ is decreased by the burrs which remain and clogging is generated in holes 56′ and 57′. Therefore, it is difficult to blow out the cooling medium from the top portion TP.
When attempting to make the cooling medium in the cavity R of moving blade 51 flow to holes 56′ and 57′ from upstream opening portions 56b and 57b, since downstream opening portions 56a′ and 57a′ are covered, a sufficient amount of cooling medium cannot be blown out to the top portion TP for cooling. If cooling of the top portion TP is not normally carried out, problems arise in that burnout and cracking are generated on the top portion TP and that the durability of the turbine is decreased.